Why did Auggie think that Jack pretended to be his friend?

From the text:

He had overheard some very mean things some boys were saying about him. He didn’t care about what the other boys had said, he expected that, but he was hurt that one of the boys was his “best friend” Jack Will. I remembered his mentioning Jack a couple of times over the past few months. I remembered Mom and Dad saying he seemed like a really nice kid, saying they were glad August had already made a friend like that.

“Sometimes kids are stupid,” I said softly, holding his hand. “I’m sure he didn’t mean it.”

“Then why would he say it? He’s been pretending to be my friend all along. Tushman probably bribed him with good grades or something. I bet you he was like, hey, Jack, if you make friends with the freak, you don’t have to take any tests this year.”